Electronic integrated circuits constituting digital electronic systems have continuously been shrinking in size since 1960s, and scientists and engineers are expected to carry on the trend of miniaturization of the integrated circuits. However, the laws of quantum mechanics and the limitations of fabrication techniques may soon prevent further reduction in the size of current conventional integrated circuits. For example, a typical process for fabricating an integrated circuit chip is to stencil the pattern of microscopic components of the chip onto silicon with the help of an ultraviolet light, which works fine for the components sized down to about 300 nanometers. However, it may become unfeasible for the components sized below about 100 nanometers.
In order to continue the miniaturization of circuit elements down to the nanometer scale, perhaps even to the molecular scale, researchers are investigating several alternatives to the conventional circuit elements. Among them, one is to design compounds that can function as logic gates, which are basic building blocks of electronic integrated circuits. The incentive for such effort is that molecules are naturally occurring nanometer-scale structures, and can be synthesized identically, inexpensively, and easily. Examples of molecular logic to date are comprised of compounds that undergo photo-induced reversible structural changes or supramolecular ionic binding upon exposure to external stimuli to elicit responses similar to logic operations. These molecular-scale logic gates may lead to the replacement of their electronic predecessors.
An exemplary compound that may be applied in the molecular electronics is shown in FIG. 1. A molecular triad 100 includes a synthetic porphyrin 110 covalently linked to both a fullerene 120 and a carotenoid polyene 130, for which one may see more detailed discussion in P. A. Liddell, et al. J. Am. Chem. Soc. 1997, 119, 1400-1405. When the porphyrin 110 absorbs light, it donates an electron to the fullerene 120. The carotenoid polyene 130 then transfers an electron to the porphyrin 110 to give a final charge-separated state. This state has a relatively long lifetime, and stores a considerable fraction of the light energy as electrochemical potential energy. This conversion of light energy to electrochemical potential is analogous to the way of which plants carry out solar energy harvesting during photosynthesis. The triad 100 is corresponding to a molecular-scale photovoltaic cell, and may function as a molecular-scale AND logic gate.
FIGS. 2 and 3 show molecular implementations of AND and OR logic circuits, respectively. As shown in FIG. 2, molecular AND logic circuit 200 has two input ports 210, input A and input B, which are connected to two polyacetylene resistors 220, respectively. The circuit 200 also has two PNP-type transistors 230. Each transistor 230 is formed with a tetracyanoquinodimethane (hereinafter “TCNQ”) and a tetrathiafulvalene (hereinafter “TTF”). Emeraldine base polyaniline 240 serves as wires to connect these components. The circuit 200 is capable of performing a logic AND function to produce a desired signal through an output port 250, in response to input signals from the two input ports 210, input A and input B.
Now referring to FIG. 3, OR circuit 300 includes rectifier diodes 310 that are formed with TCNQ and TTF, two input ports 305, A and B, which are coupled with the rectifier diodes 310, and an output port 350. Emeraldine base polyaniline 320 serves as wires to connect the diodes 310 to polyacetylene resistors 330, respectively.
In addition, nanotubes have high thermal conductivity and are stable at high temperatures, E. Lerner, The Industrial Physicist, December 1999, 22-25, and capable of carrying higher currents than either copper or superconductors. Nanotubes could become an important component in the molecular electronics. For example, a room-temperature single-electron transistor (SET) within a single carbon nanotube has been reported, H. W. C. Postma, et al., Science, 2001, 293, p. 76, which can be switched between state of “on” and state of “off” with a single electron and works efficiently at room temperature.
However, challenges are to devise molecular structures that act as electrical components, and to combine these molecular structures into a more complex circuit structure needed for digital electronic systems.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.